Thermally stable thiosulfate hydroxyethylcellulose suspension

Abstract: A storage and thermally stable polymer suspension in fresh water remains pourable after mixing at 6 months comprises at least 30% ammonium or sodium thiosulfate and 20% hydroxyethylcellulose. In an oil drilling fluid, yield value decreases less than 35% when heated for 16 hours at 250°F.

Description: The invention relates to aqueous salt suspension of polymer. In particular the invention relates to a thermally, stable suspension of hydroxyethylcellulose in ammonium or sodium thiosulfate. Fluidized polymer suspensions which employ an inorganic salt are known from U.S. patents 4,283,229, 4,883,536 and 4,883,537. These salts include alkali metal and ammonium ion carbonates, phosphates and sulfates. EP-A 482533, US-A 5,228,908 and US-A 5,228,909 employ sodium formate as the suspending salt. BE-A 447 906 is directed to molding materials comprising cellulose ethers. This document discloses that the presence of some sulfur containing compounds such as sodium hyposulfite avoids the discoloration of cellulose ethers during the molding of such materials under pressure and at high temperature. US-A 5,028,263 discloses stable aqueous suspensions of water-soluble polymers comprising a water-soluble polymer dispersed in an aqueous solution of an ammonium salt having a multivalent anion. Some of the water-soluble polymers used in this document such as hydroxyethylcellulose are difficult to incorporate into an aqueous system and therefore it is suggested to apply the water-soluble polymer in form of an aqueous suspension. Such an aqueous suspension is said to be stable and can comprise a high amount of water-soluble polymer, if it comprises a specific ammonium salt. It is an object of the invention to provide thermal and storage stable aqueous suspensions of hydroxyethylcellulose containing at least 30% by weight ammonium or sodium thiosulfate. The suspensions are prepared by dissolving the ammonium or sodium thiosulfates in water and dispersing the hydroxyethylcellulose into the solution. The suspensions are prepared with fresh water and remain pourable after mild stirring for six months. It has been discovered that hydroxyethylcellulose (well known as a nonionic thickening agent) can be protected against thermal degradation by thiosulfate salts. Hydroxyethylcellulose is available from the Aqualon Company as Natrosol® hydroxyethylcellulose (HEC). Grade HHXR is of high molecular weight. Grade EXR is low molecular weight. HEC is also available from Union Carbide and Daicel. HEC is widely used as a paint thickener and for other applications in food, cosmetics and pharmaceuticals. Ammonium thiosulfate is a commodity chemical which is generally believed to be interchangeable with sodium thiosulfate. For example in photographic processing, "fixer" is usually sodium thiosulfate, while ammonium thiosulfate can also be substituted if desired. The invention has industrial applicability for oil drilling fluids for deep wells where high temperatures create problems for existing drilling fluids. The following examples illustrate the practice of the invention. Examples 1 and 2 An ammonium thiosulfate brine was prepared by dissolving 500g ammonium thiosulfate in 500g water. While 750g of this solution was stirred with a Hamilton Beach mixer at 11,000 rpm, a 250g portion of Natrosol® hydroxyethylcellulose was added into the solution over 20 minutes. The procedure was repeated with a different HEC. Table 1 gives Brookfield LVF (25°C) viscosity results over time for the two grades of Natrosol® HEC tested in the suspensions of the invention. EX HEC t-o t-1 day 1 week 1 month 2 months 1 250 HHXR 260 230 210 150 240 2 250 EXR 165 215 180 160 200 Both suspensions containing 25 wt. % HEC and 37.5 wt. % ammonium thiosulfate were stable and remained pourable after mild stirring even after six months storage. Examples 3 and 4 Table 2 gives the Brookfield LVF (25%) viscosities of similar suspensions prepared by the procedure described in Example 1, using either ammonium thiosulfate or sodium thiosulfate with different samples of HEC 250 HHXR (Examples 3 and 4). EX HEC t-1 day 1 week 1 month 2 months 3 Ammonium Thiosulfate 140 175 145 145 4 Sodium Thiosulfate 390 235 265 255 Although brine viscosities were different in both cases, reflecting a difference in solubility of the HEC in both brines, the viscosities of the suspensions were stable for two months in each case. Example 5 This example illustrates the efficiency of ammonium thiosulfate as a thermal stabilizer for a sea water polymer solution. HEC was added to sea water in the amount of either 1 or 2 pounds per barrel (ppb); (one ppb is equivalent in metric units to 2,847 kg/m 3 ), with and without 30% by weight ammonium thiosulfate (A.T.). Yield values of lb/100 ft 2 = 0.5 Pa) were determined initially for these combinations and then after 16 hours at 250°F (122°C). Table 3 gives comparative results in which the decrease in yield value is expressed as a percentage. HEC Conc. (ppb) % A.T. w/w HEC Initial Y.V. 16 hrs./250°F Y.V. % Y.V. Drop 1 0 6 4.5 72 1 30 13 9 31 2 0 45 10 78 2 30 40.5 37 9 These results show the surprising degree of thermal protection provided by the simple addition of ammonium thiosulfate whereby it is also possible to prepare storage stable suspensions as illustrated in Example 1. Example 6 Tests were run similar to Examples 1-4 except that both ammonium thiosulfate and sodium thiosulfate were used in addition to other components, as shown in Table 4 below. In these tests, Natrosol® 250 HHRP hydroxyethylcellulose available from Aqualon was used as the polymer. A.T. = Ammonium Thiosulfate, S. T. = Sodium Thiosulfate. Example 7 Table 5 summarizes the effects of addition of HEC suspensions in thiosulfates, as percent drop V after 16 hours at 250°F in properties. After 16 Hours at 250°F Percent Drop In Apparent Viscosity Plastic Viscosity Yield Point Medium A.T. S.T. A.T. S.T. A.T. S.T FW (fresh water) 86 88 79 86 96 92 FW + NaOH (1 ppb) 15 56 0 33 34 81 FW + MgO (1 ppb) 22 19 7 0 39 42 SW (4% NaCl) 89 89 86 86 92 92 SW + NaOH (1 ppb) 11 46 0 23 32 75 SW + MgO (1 ppb) 18 15 0 0 33 32 Conclusions from Table 5: 1. In FW and SW alone, neither ammonium thiosulfate (A.T.) nor sodium thiosulfate (S.T.) provided thermal stability to HEC in suspension form by any of three indicators (Apparent Viscosity, Plastic Viscosity or Yield Point). 2. With 1 ppb NaOH present, A.T. is a more effective thermal stabilizer than S.T. in both FW and SW. Both provide some thermal stability. Plastic viscosity indicates this best. 3. The best thermal stability is provided with 1 ppb MgO present, in either FW or SW. Both S.T. and A.T. are about equally effective, with S.T. being slightly preferred. The best indicator is Plastic Viscosity or Apparent Viscosity. Example 8 Table 6 summarizes the effect of addition of 30% solid thiosulfate on thermal stability of HEC in different media, after 16 hours at 250°F. After 16 Hours at 250°F Percent Drop In Apparent Viscosity Plastic Viscosity Yield Point Medium A.T. S.T. A.T. S.T. A.T. S.T. FW (fresh water) - - - - - - FW + NaOH (1 ppb) 80 84 67 75 95 95 FW + MgO (1 ppb) 28 27 0 0 50 59 SW (4% NaCl) - - - - - - SW + NaOH (1 ppb) 66 78 50 61 37 95 SW + MgO (1 ppb) 30 24 8 0 50 55 The concentration of the hydroxyethylcellulose is about 25% in each of the different media. 1. With 1 ppb NaOH present, in FW neither A.T. nor S.T. provide much thermal stability. In SW, A.T. provides somewhat better stability than S.T. Yield Point indicates the best stability. 2. The best thermal stability is provided with 1 ppb MgO present, both in FW and SW. Both S.T. and A.T. are equally effective. Plastic Viscosity and Apparent Viscosity are the best indicators. The Brookfield viscosities were measured according to general standard practice (1% solution at 25°C). If in the present invention magnesium oxide and sodium hydroxide respectively is used, the concentration thereof will range preferably from 0.05 to 2.0 pounds per barrel (ppb).

Claims: An aqueous suspension of hydroxyethylcellulose in ammonium or sodium thiosulfate wherein the ammonium or sodium thiosulfate comprises at least 30% by weight of the total weight of the suspension. The suspensions of Claim 1 where the hydroxyethylcellulose comprises at least 20% by weight of the total weight of the suspension. The suspensions of Claim 2 where the ammonium or sodium thiosulfate comprises at least 35% by weight and the hydroxyethylcellulose comprises at least 25% by weight of the total weight of the suspension. The suspensions of Claim 3, further characterized in that the suspensions remain thermally stable and pourable after stirring for up to 6 months. A thermally stable oil drilling fluid comprising at least 2.847 kg/m 3 (1 ppb) of hydroxyethylcellulose and at least 30% by weight ammonium or sodium thiosulfate based on the weight of hydroxyethylcellulose. The fluid of claim 5 further comprising magnesium oxide. The fluid of claim 5 further comprising sodium hydroxide. A process for preparing a thermally stable suspension of hydroxyethylcellulose comprising the steps: (1) dissolving ammonium or sodium thiosulfate in water to prepare a stirred solution; and (2) adding hydroxyethylcellulose to the stirred solution to prepare a suspension of hydroxyethylcellulose. The process of claim 8 whereby in step (1) the thiosulfate comprises at least 35% by weight of the total weight of the suspension. The process of Claim 9 whereby in step (2) the hydroxyethylcellulose comprises at least 25% by weight of the total weight of the suspension. The process of claim 10 where the suspension remains thermally stable and pourable after stirring for up to six (6) months.

Inventors: LUKACH CARL ANDREW, ZAPICO JOSE, LUKACH, CARL ANDREW, ZAPICO, JOSE
Applicants: AQUALON CO, AQUALON COMPANY
Application year: 1994
Main classification: C08L1
IPC codes: C09K8, C08K3, C08J3